The present invention generally pertains to the processing of video signals derived from a film having successive image frames, such as motion picture film, and is particularly directed to determining the sequential position of video fields in the derived video signal.
A large number of video signal sources are derived from commercial motion picture film, which is filmed at twenty-four frames per second. The transfer of imaging information from 24 frame-per-second film to 60 field-per-second video is typically performed using storage tube scanning, flying-spot scanning, or more recently, line scanning with a frame store and digital sequencing. In spite of the difference in film-scanning methods, the frame-rate conversion techniques are the same. In practice, the most common frame-rate conversion scheme for film to video transfers is the 3:2 pulldown method film.
FIG. 1 shows the derivation of interlaced video from film using this method. The images were recorded on the film 10 at the rate of 24 frames per second; and interlaced odd video fields 11 and even video fields 12 were derived from the film 10 at the rate of 60 fields per second.
In the repetitive sequentially varying relationship of the received video fields 11, 12 to the film frames 13 from which they were derived in accordance with the 3:2 pulldown method, ten video fields 11, 12 are derived from the first film frame, the second video field being an even field derived from the first film frame, the third video field being an odd field derived from the first film frame, the fourth video field being an even field derived from the second film frame, the fifth video field being an odd field derived from the first film frame, the sixth video field being an even field derived from the third film frame, the seventh video field being an odd field derived from the third film frame, the eighth video field being an even field derived from the third film frame, the ninth video field being an odd field derived from the fourth film frame, and the tenth video field being an even field derived from the fourth film frame.
Referring to FIG. 1, it is seen that the odd and even video fields in the derived video signal are produced in a predetermined repetitive sequentially varying relationship to the film frames. In processing the derived video signal it often is necessary to know the sequential position of video fields in the derived video signal.